Apparatus for data acquisition and application in an injection molding process

ABSTRACT

An injection molding apparatus utilizing an injection molding machine having a pair of separable mold elements forming a mold cavity therebetween and arranged for separation for the removal of the molded product, means for opening and closing the mold elements, and injection means for injecting the moldable material into the mold cavity at a first pressure and for exerting a second pressure on the moldable material as it cures in the cavity. The apparatus provides data acquisition and application in the operation of the injection machine. The apparatus includes means for measuring the separation of the mold elements during the injection of the moldable material, and means for causing the injection pressure from the first pressure to the second pressure upon detecting a predetermined separation known to produce an acceptable product. Means is provided for maintaining the second pressure until the thermoplastic material has cured. The mold is then opened and the cured product is ejected. Means is provided for detecting and accumulating the maximum separation of the mold elements during a plurality of mold cycles while producing acceptable product. Then, a value for the separation of the mold elements is selected based on the maximum measurements accumulated, and means is provided for substituting the new value for use during subsequent cycles to improve product quality for the predetermined separation previously used.

RELATED APPLICATIONS

The present application is related to my co-pending applications Ser.Nos. 115,675 and 115,676, both filed on Nov. 2, 1987, and now U.S. Pat.Nos. 4,767,300 and 4,767,579, respectively, and to my co-pendingapplications entitled METHOD OF CONTROLLING AN INJECTION MOLDINGPROCESS, Ser. No. 208,903, APPARATUS FOR CONTROLLING AN INJECTIONMOLDING PROCESS, Ser. No. 209,140, METHOD OF IMPROVING THE CYCLE TIME OFAN INJECTION MOLDING PROCESS, Ser. No. 209,405, APPARATUS FOR IMPROVINGTHE CYCLE TIME OF AN INJECTION MOLDING PROCESS, Ser. No. 208,439, METHODOF GENERATING AN ACCEPTABLE PART AND PROCESS WINDOW FOR AN INJECTIONMOLDING PROCESS, Ser. No. 208,908, APPARATUS FOR GENERATING ANACCEPTABLE PART AND PROCESS WINDOW FOR AN INJECTION MOLDING PROCESS,Ser. No. 208,909, METHOD OF OPERATING AN INJECTION MOLDING PROCESSUTILIZING AN ACCEPTABLE PART AND PROCESS WINDOW, Ser. No. 208,928,APPARATUS FOR OPERATING AN INJECTION MOLDING PROCESS UTILIZING ANACCEPTABLE PART AND PROCESS WINDOW, Ser. No. 208,927, and METHOD OF DATAACQUISITION AND APPLICATION FOR AN INJECTION MOLDING PROCESS, Ser. No.208,918, all filed on even date herewith.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to a apparatus for data acquisitionand application in an injection molding process whereby the process andresulting product may be closely controlled.

2. Description of the Prior Art

The production of consistent and uniform product by the injectionmolding process has been a long standing objective of the injectionmolding industry. This objective has become more difficult to achieve asmore and more products are produced by this process which are everincreasingly complex and have ever tighter tolerances. The objective hasbeen further complicated by the trend towards fewer and fewer operatorsassociated with the injection molding machines, requiring greaterautomatic control of the process and apparatus.

A variety of interrelated parameters of the material, machine, and moldmust be accommodated by any automated control system for thesatisfactory operation of an injection molding apparatus; among theseparameters are the type of material being molded, the consistency of theplastic characteristics, the molding cycle time, the machine shot size,melt viscosity and temperature consistency, mold clamp pressure, andinjection pressure, among others. It has been found that, as each ofthese parameters varies during the operation of an injection moldingmachine, the product uniformity may suffer without constant operatorattention. One of the solutions to the control of these many variableshas been the use of longer molding cycles to assure that the effect ofthese variations are accommodated without adversely affecting the finalproduct. As a result, the productivity of the machines have been reducedfrom that otherwise possible in order to obtain consistentlysatisfactory parts.

Many attempts have been made at automatically controlling the injectionmolding process to produce uniform and consistent product, yet none ofthe proposed solutions has gained widespread acceptance, at leastpartially due to the fact that the proposed solutions do notsatisfactorily address all of the variables involved. Moreover, few ofthe prior art controllers even address the issue of cycle timeimprovement.

Among the solutions proposed are those taught by U.S. Pat. Nos.2,433,132 and 3,976,415 and French Pat. No. 2,527,976 wherein theseparation of the mold elements at the part-line is measured and theresult utilized to change the machine from the injection phase to apulsing of the injection ram to maintain the part-line separationconstant during the packing and curing of the mold. This system has notbeen found to be feasible for the manufacture of small, precision partssince it is nearly impossible to so precisely control the pulsing of theram to pump the microscopic amounts of material necessary to achieve thedesired volumetric accuracy of such parts. This process is alsodescribed in the article by Bernard Sanchagrin, entitled "ProcessControl of Injection Molding" in the mid-Jun., 1983 issue of PolymerEngineering and Science, Vol. 23, No. 8.

Other proposed solutions are exemplified by U.S. Pat. Nos. 2,671,247 and3,859,400, which teach the control of the switch point of the injectionmolding machine by sensing the pressure within the mold itself. Thissystem proposes sensing the pressure within the mold to shift themachine from the injection phase to the holding phase while the materialcures within the mold. The measurement of the pressure within the molddoes not satisfactorily reflect the multiple variables noted above. Forexample, if the material characteristics are held constant and themachine clamp force is allowed to vary, even if the pressure in the moldcavity remains constant, the part size produced will vary with the clampforce, increasing with reduced clamp force and vice versa. Similarly,with variations in material characteristics, if the viscosity of thematerial changes, as it may with changes in the injection temperature orwith different batches of material, even if the amount of materialinjected into the mold is substantially constant, the varying viscositywill affect the resistance of the material to being forced into the moldcavity and thus the pressure transmitted into the mold cavity from theinjection ram. Accordingly, it will be seen that measuring the pressurein the mold cavity will not truly reflect many of the variables thatinfluence the process and the product.

Another proposed method of controlling injection molding processes isthat taught by U.S. Pat. No. 3,940,465 wherein the measurement of theseparation of the part-line is utilized to control the cure time of theinjection molding cycle. This type of control fails to reflect all ofthe variables, noted above, which must be accommodated to accuratelycontrol part weight and dimension.

U.S. Pat. No. 4,135,873 teaches the measurement of the part-lineseparation and comparing the separation with a predetermined value andthereafter varying the injection pattern of the injection ram during thefollowing molding cycle. This system does not provide control of theprocess on a real time basis, reflecting system conditions that areaffecting the current cycle. Such a system merely reflects what occurredon the previous cycle, resulting in a tendency for the system to huntrather than zero into a mode of operation which provides productconsistency.

U.S. Pat. No. 4,131,596 teaches the measurement of the part-lineseparation to reduce the mold clamping pressure upon the measurement ofa predetermined separation to minimize any damage to the mold due toflashing of the material at the part-line. This, of course, does notcontribute to the control of product weight and dimension.

Japanese Patent Publication No. 11974 of 1978 discloses a method ofcontrolling an injection molding machine wherein the part-lineseparation is measured and, upon reaching a predetermined referenceseparation, the machine is switched from a material filling mode to adwelling mode. The mold separation is then measured and the maximumseparation is determined. Thereafter, pressure during the dwell orcuring phase of the mold cycle is controlled dependent upon the maximumseparation reached to control the final mold separation value at the endof the cure time. Thereafter, the reference separation value for theswitch point for the following cycle is changed to accommodate thevariations in the machine operation detected during the first cycle.This system of control has the disadvantage that the switch point isdetermined by the preceeding cycle and thus does not reflect theconditions of the current cycle. This system of control thereafterattempts to adapt to the variations in the molding conditions existingduring the current cycle by controlling the holding pressure during thecure phase of the cycle which can adversely affect part weight anddensity uniformity.

Each of the foregoing control systems has either been too complex andexpensive and/or has not provided the requisite control related to allof the variables acting upon an injection molding process.

It has been found that the variables noted above are reflected in themolding process through the part-line opening which also reflects theconsistency of the dimensions and weight of the molded product. Duringrepeated molding cycles variations in the product from the moldingprocess can range from under-filled mold cavities (short) to over-filledmold cavities (flashed). The aim point for the process will be somewherebetween these two extremes to produce a product which meets thedimensional and weight tolerances established for that process andproduct. Further, it is known that mechanical and thermal strains areinherent in the molding process which are then transfered to the moldedproduct, sometimes to the detriment of the dimensional stability andlife of the product. The mechanical strains are produced by the clampingpressure necessary to hold the mold elements together against the forceof the injection of the molten material. The thermal strains occurduring the filling and packing of the mold with the high temperaturemolten material and its effect on the much cooler mold cavity walls,followed by the shrinkage of the material as it cures.

Accordingly, the provision of apparatus for controlling and improvingthe cycle time of an injection molding process without the requirementof constant operator attention would significantly enhance theproductivity and applicability of the process, as well as minimize thestrain on the molding apparatus and the residual stress in the moldedproduct.

SUMMARY OF THE INVENTION

Thus, the present invention provides apparatus which provides dataacquisition and application in the operation of an injection moldingmachine having a pair of separable mold elements forming a mold cavitytherebetween and arranged for separation for the removal of the moldedproduct, means for opening and closing the mold elements, and injectionmeans for injecting a moldable material into the mold cavity at a firstpressure and for exerting a second pressure on the moldable material asit cures in the cavity. The apparatus includes means for measuring theseparation of the mold elements during the injection of the moldablematerial, and means for changing the injection pressure from the firstpressure to the second pressure upon detecting a predeterminedseparation known to produce an acceptable product. Means is provided formaintaining the second pressure until the thermoplastic material hascured. The mold is then opened and the cured product is ejected. Meansis provided for detecting and accumulating the maximum separation of themold elements during a plurality of mold cycles while producingacceptable product. Then, a value for the separation of the moldelements is selected based on the maximum measurements accumulated, andmeans is provided for substituting the new value for use duringsubsequent cycles in place of the predetermined separation valuepreviously used.

Further, the present invention provides a method wherein the selectedvalue that is substituted for the predetermined value is set at 30-70%of the greatest maximum separation measurement accumulated.

More particularly, the present invention provides a method wherein theselected value that is substituted for the predetermined value is set at50% of the greatest maximum separation measurement accumulated.

Various means for practicing the invention and other features andadvantages thereof will be apparent from the following detaileddescription of illustrative preferred embodiments of the invention,reference being made to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevation view, partially in section, of an injectionmolding apparatus adapted for use with the present invention;

FIG. 2 is an enlarged portion of the part-line between the mold elementsshowing the mounting of the part-line sensor;

FIG. 3 is a graph illustrating the part-line separation between moldelements during a complete molding cycle;

FIG. 3a is a graph similar to that of FIG. 3 showing an effect ofvarying the control point of the cycle; and

FIGS. 4-1 to 4-4 is a logic chart illustrating the operation of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

One form of an injection molding apparatus 10 is illustrated in FIG. 1and comprises a pair of pressure platens 12 and 14 arranged to carry apair of mold elements 16 and 18, respectively. The mold elements arearranged to meet at a part-line 20 and form a mold cavity 22therebetween, all in a manner well known in the art. Platen 12 and themold element 16 associated therewith are stationarily arranged on themachine while platen 14 and mold element 18 associated therewith aremovably arranged to be displaced along tie bars 24 and 26 between anopen and closed position by a hydraulic cylinder 28.

A plastic extruder assembly 30 is arranged to engage a gate 32 in themold element 16 with an injection nozzle 34 at the outlet end of theextruder. The main portion of the extruder comprises an extruder barrel36 having a rotating plasticating screw 38 therein which receivesparticulate material from a supply 40, and via heat and manipulationplasticates the material for injection through nozzle 34 into the moldcavity 22. To aid in the plastication of the material the extruderbarrel is provided with encasing heater elements 42 and 44, in a mannerwell known in the art. The screw is rotated by a gear 46 driven by amotor, not shown, and is driven longitudinally to inject the moltenmaterial into the mold cavity by means of a hydraulic cylinder 48. Thehydraulic cylinder 48 is provided with hydraulic fluid from a powersource in order to drive the screw longitudinally during the injectionprocess. The hydraulic fluid supply provides both a high pressure forthe injection phase of the cycle as well as a low pressure for theholding phase of the cycle, as is well known in the art. One example ofsuch a hydraulic supply comprises two separate sources of hydraulicfluid, one a high, injection pressure 50, and the other a lower, holdingpressure 52 connected by lines 54 and 56 to a control valve 58 whichdetermines which pressure is supplied, by line 60, to the hydrauliccylinder 48.

While a reciprocating screw injection molding machine is illustrated forthe purposes of describing the present invention, it will be appreciatedby those skilled in the art that other forms of injection moldingmachines such as plunger and transfer-compression molding machines mayalso be employed.

A distance sensor 62 is mounted on the stationary mold element 16adjacent part-line 20. A distance sensor target 64 is mounted on themovable mold element 18 in opposition to the sensor 62. The target maycomprise an adjustable bolt or pin member 66 which is arranged toprovide the target for sensor 62. The sensor and target are arranged tocome into close proximity when the mold elements are closed and clampedbut are carefully positioned so that at no time do they contact oneanother. The sensor element 62 may be of any type known in the artincluding capacitive, inductive, optical, or other type proximity sensorhaving a substantially linear output over a range from +10 volts to -10volts representing a distance range of 0.020 inches. The proximitysensor 62 provides an analog output signal via line 68 to a centralprocessing unit, or controller 70, the operation of which will bedescribed herein below. The controller 70 is arranged to provide anoutput signal via line 72 to actuate a portion of the molding apparatus,such a valve actuator 74, which is connected to valve 58. Thus, whencontroller 70 receives the appropriate signal from the proximity sensor62, it provides an output to valve actuator 74 which switches the valve58 from the high injection pressure 50 to the lower, holding pressure 52to thereby control the cycle of the injection molding machine inaccordance with the present invention.

It has been found that when an injection molding machine is operatedwith a sensor sufficiently sensitive to accurately measure the part-lineseparation between the mold elements 16 and 18, that a characteristictime/displacement (separation) curve is generated for that machine. Ithas also been found that the part-line separation dimension representedby this curve reflects and integrates the multiplicity of variablesoperating on the molding machine during the current molding cycle. Thesevariables include mold and machine rigidity, clamp pressure variations,friction and inertia in the mold clamping system, the machine shot size,melt viscosity and temperature consistency, the characteristics of theplastic being molded, and the characteristics of the mold and runnersystem employed.

One example of such a time/displacement curve is illustrated in FIG. 3and will be referred to in the following description of the operation ofthe present invention in conjunction with the logic chart illustrated inFIGS. 4-1 to 4-4. The curve represents the variation of the part-lineseparation with respect to time during a single molding cycle of aninjection molding machine operating at a steady state condition afterstabilization following start-up. As the empty mold elements begin toclose, the part-line sensor will start to indicate the part-lineseparation as the mold elements approach each other. As the moldelements approach a predetermined separation S₁, which is designated theEntry Threshold, for example a separation of 0.0095 inches, the highpressure clamp system on the injection molding press is disabled,actuating the low-pressure protection portion of the system. As theEntry Threshold is crossed by the continued closing of the moldelements, the master counter/timer is actuated at T₁ and a "bad partsort" output signal is turned off or disabled. As the mold elementscontinue to close, the part-line separation reaches a "low pressureprotection" separation S₂ and the machine high-pressure clamp isenabled, permitting the high-pressure clamping of the mold so that themolten plastic material may be injected into the mold cavity.Thereafter, the timer reaches the mold closed offset point T₂, and thepart-line separation sensor is read to determine the actual measurementof part-line separation sensed after final closing and clamping of themold elements at S₃. The separation value S₃, for example 0.005 inches,is then stored in the controller memory for use later in the program.After it is determined that the mold elements have been clampedtogether, the injection of the molten material into the mold cavity isinitiated with the injection ram 30 operating under the high injectionpressure 50. As the mold fills during the time from T₂ to T₃, thepart-line separation value remains substantially constant at S₃ untilthe mold has been filled and the injection ram begins to pack out themold. At this time T₄, the mold elements begin to separate along thepart-line. When the part-line separation reaches a predetermined controlpoint S₄, a control signal is delivered to the valve operator 74,switching valve 58 from the high injection pressure 50 to the lowerholding pressure 52.

Thereafter, because of the finite lag in the signal activating the valve58 and the injection ram responding to the change in pressure, as wellas other inertial factors in the machine, the part-line separation willcontinue to increase until it reaches a maximum S₆. At that point thematerial in the mold will begin to cool and shrink and the part-lineseparation will fall back to approximately the initial mold closedseparation S₃ during the curing or cooling phase of the molding cycle.Thereafter, the mold is opened and the part ejected, at which point thesensor will indicate that the part-line separation has exceeded the ExitThreshold S₈. When the maximum part-line separation value S₆ is detectedand measured, the part-line value is calculated by subtracting from themaximum part-line separation S₆ the mold closed separation value S₃giving a part-line separation value for each part produced by the mold.Inasmuch as the mold closed separation value is determined for everycycle, variations in the performance of the machine is accommodated byrezeroing the mold closed value for every cycle of the machine.

Examples of other part-line separation curves occurring on theillustrative machine are also illustrated in FIG. 3 as dotted anddash-dot lines. These curves illustrate variations in the part-line thatmay occur because of variables acting upon the overall system. Forexample, should some minor change slightly reduce the clamping pressureduring a given molding cycle, the maximum part-line separation mightincrease as illustrated by the dotted line. So long as the increase inpart-line separation does not exceed the operator-selected maximumseparation S₇, then the part produced will still be consideredacceptable. Likewise, should the mold clamp force, as an example, beincreased during a mold cycle, the final part-line separation would bereduced as illustrated by the dash-dot line. Again, so long as the finalpart-line separation is above the operator-selected minimum separationS₅ the part will be considered acceptable. On the other hand, should themaximum part-line separation achieved fall outside the maximum andminimum part-line separation values selected by the operator, then a"bad part sort" output signal will be generated by the controller whichmay be utilized by the system or by the operator to identify or removethe out-of-specification part.

The process is initiated after the sensor has been installed in themolding machine and the controller wired to the appropriate controllingportion of the machine. The injection molding machine is then operatedby a competent operator to produce acceptable parts. During thisoperation the controller is set to the "monitor mode" whereby part-lineseparations are measured and monitored with appropriate data beingretained in the controller memory. Appropriate adjustments are made tothe process by the machine operator to achieve satisfactory moldedparts. As the satisfactory parts are identified, the part-lineseparation measurements made for those satisfactory parts are utilizedto determine the desired predetermined control point as well as themaximum and minimum times within which the maximum part-line separationoccurs while still producing satisfactory parts. At the same time, theappropriate time offsets are selected by the operator according to theparticular characteristics of that individual machine. As soon assufficient data has been collected to ensure the operator that thesampling is representative, and the values have been set into thecontroller, the controller may be switched to the "control mode" whereinit commences the control of the injection molding process.

Referring now to FIG. 3a wherein a part-line separation graph similar tothat of FIG. 3 is illustrated, the effect of varying the selection ofthe control point on the cycle time is illustrated. For example, shoulda lower control point S₄ ' be selected, occurring at an earlier time T₄', the resulting part-line separation following the control point wouldbe as represented by the dotted line and would result in a lower maximumpart-line separation S₆ '. As illustrated, should such a lower controlpoint be selected, the part produced would still fall within thepreselected maximum/minimum and thus fall within the acceptable partquality window with a finite reduction in the cycle time required toreach the maximum part-line separation S₆ ' and to produce theacceptable part.

As a comparison, if a control point S₄ " is selected which is greaterthan that previously utilized, the resulting part-line separationfollowing the control point would be represented by the dash-dot lineand the maximum part-line separation S₆ " would be increased. Thus, theoperator has the option of increasing or decreasing the average cycletime by adjusting the preselected control point.

The various timing offsets (e.g., T₂ and T₃ in FIG. 3) provided by thepresent system permits an operator to select the appropriate timingintervals appropriate to that particular machine. Moreover, the timingoffsets also function as blanking signals to limit the reading of thepart-line separation sensor to the appropriate period in the cycle.Thus, any vibration or jitter present in the machine that occurs duringthe blanked portions will not provide spurious part-line separationsignals that could adversely affect the overall machine control.

The present invention is particularly directed to a method of acquiringand applying data to the molding process to facilitate the establishmentof the process. After the molding machine has been operating stably andhas been producing satisfactory parts, the data accumulation isinitiated and the maximum separation of the mold elements is detected,measured and accumulated from a plurality of mold cycles. (Ten cycleshave proven to provide an adequate sample.) Then, the operator selects anew value for the control point based on the maximum measurementsaccumulated. It has been found that acceptable maximum part-lineseparations can be achieved when the control point is set to a valuebetween 30 and 70% of the highest maximum part-line separation measuredduring the accumulation period. More particularly, it has been foundthat setting the control point at 50% of the highest maximum part-lineseparation generally provides optimum performance. The new control pointvalue is the substituted for the control point previously used.

Accordingly, the present invention provides a method for acquiring andapplying data for use in controlling an injection machine and process byusing the measurement of the separation of the mold elements as averification of achieving product quality while improving the cycletime. The control and measurement of the part-line separation assurespart completion as well as part uniformity and quality. The presentinvention provides verification that all of the variable parameters inthe molding machine and process are combining to achieve the specifiedpart. Moreover, the present method obviates unstable operation that canoccur when each cycle variation is reflected in the next cycle. Eachcycle is controlled by the system conditions existing during that cycle,yet adaptation to longer range trends is provided to maintain partconsistency.

The invention has been described in detail with particular reference toa presently preferred embodiment, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

What is claimed is:
 1. In an injection molding machine having a pair ofseparable mold elements forming a mold cavity with a part-linetherebetween, means for opening and closing the mold elements, andinjection means for injecting a moldable material into said mold cavityat a first pressure causing the mold elements to separate and forexerting a second pressure on said moldable material as it cures in saidcavity, the improvement comprising apparatus for data acquisition andapplication in said injection machine comprising means for measuring theseparation of the mold elements during the injection of the moldablematerial, means for changing the injection pressure from said firstpressure to said second pressure upon detecting a predeterminedseparation known to produce an acceptable product, means for maintainingsaid second pressure until the thermoplastic material has cured, saidseparation measuring means arranged to continue the measurement of theseparation of the mold elements following the changing of the pressure,means for sensing when the separation of the mold elements stopsincreasing and registering the cessation as the maximum separation ofsaid mold elements, means for accumulating said maximum separations froma plurality of mold cycles while producing acceptable product, means forthen selecting a value for said separation of said mold elements basedon the maximum separations accumulated, and means for substituting saidselected value of said separation for said predetermined separation foruse during subsequent cycles to improve product quality.
 2. Theapparatus according to claim 1 wherein said maximum separations areaccumulated over ten mold cycles.
 3. The apparatus according to claim 1wherein said selected value that is substituted for said predeterminedvalue is substantially equal to 30-70% of the greatest maximumseparation accumulated.
 4. The apparatus according to claim 1 whereinsaid selected value that is substituted for said predetermined value issubstantially equal to 50% of the greatest maximum separationaccumulated.
 5. In an injection molding machine having a pair ofseparable mold elements forming a mold cavity with a part-linetherebetween, means for opening and closing the mold elements, andinjection means for injecting a moldable material into said mold cavityat a first pressure causing the mold elements to separate and forexerting a second pressure on said moldable material as it cures in saidcavity, the improvement comprising apparatus for data acquisition andapplication in said injection molding machine comprising means formeasuring a separation of said mold elements during the closing of themold elements, means for sensing a predetermined separation of said moldelements during the closing of the mold and initiating a first timingcycle of a predetermined time length in response thereto, means forsensing a termination of relative closing movement between said moldelements and determining whether or not said termination occurs duringsaid first timing cycle, means for reopening said mold elements if saidtermination does not occur during said first timing cycle and, if saidtermination occurs during said timing cycle, means for commencing theinjection of a moldable material into said mold cavity at said firstpressure and initiating a predetermined time window starting apredetermined time after said injection commencement, said measuringmeans arranged to measure the separation of the mold elements during theinjection of the moldable material, means for detecting a predeterminedseparation of said mold elements and determining whether said separationoccurs within said time window, means for changing the injectionpressure from said first pressure to said second pressure upon detectingsaid predetermined separation and for maintaining said second pressureuntil the thermoplastic material has cured, means for detecting amaximum separation of said mold elements and for determining whether ornot said maximum separation is within limits by being greater than apredetermined minimum separation and less than a predetermined maximumseparation, means for accumulating said maximum separation measurementsthat are within said limits from a plurality of mold cycles whileproducing acceptable parts under the control of an operator, means forselecting a predetermined separation of said mold elements based on themaximum separation measurements accumulated, and means for substitutingsaid selected value of said separation for said predetermined separationfor use during subsequent cycles to improve product quality.